This invention relates generally to the field of liquid level sensing and control and specifically to a positive temperature coefficient sensor and associated control circuit for a clothes washing machine.
Thermal water level detectors have been used in a variety of configurations. Generally, the electrical resistance of the sensing element varies as a function of temperature. The element is mounted in the water container so that when the water covers the element, its temperature, and therefore its resistance, changes. This change in resistance is detected by a circuit which provides an indication that the water has reached the level of the sensor. Various changes to this basic configuration which improve the thermal characteristics and accuracy have been disclosed.
Positive Temperature Coefficient resistors are commonly used as sensing elements. A PTC resistor is a device that increases in electrical resistance as its temperature increases. As a heater it will self-regulate its own temperature and will not go into thermal runaway.
One disadvantage of this type of circuit is that only one water level can be detected. In order to detect more levels, other sensors and associated circuits must be added at each level to be measured.